1. Field of the Invention
The present invention relates to photopolymerizable compositions, to flexographic printing plates derived from said compositions and to flexographic printing relief forms prepared from said plates. More in particular, the present invention relates to compositions showing an improved combination of processing stability, transparency and ozone resistance.
2. Description of the Related Art
Photopolymerizable printing plates are known for use in making flexographic printing forms. The printing surface is produced by exposing a photopolymerizable layer image-wise to actinic radiation and subsequently removing the unexposed, non-photopolymerized areas of the printing plate. Examples are found in the following patents: DE C-2215090, U.S. Pat. Nos. 4,266,005, 4,320,188, 4,126,466, 4,430,417, 4,460,675 and 5,213,948.
Such photopolymerizable printing plates usually comprise a support, an optional adhesive layer or other underlayer, one or more photopolymerizable layers, an optional elastomeric intermediate layer and a cover layer. A preferred method for making such multilayer photopolymerizable printing plates is by a process in which a previously extruded photopolymerizable composition is fed into the nip of a calendar and is calendered between a support layer and a cover layer, thus forming a photopolymerizable layer between them. EP B-0084851 disclosed a preparation method for a multilayer photopolymerizable printing plate, having an added elastomeric layer between the cover layer and the photopolymerizable layer.
The photopolymerizable layers contain polymeric binders, photopolymerizable monomers, photo-initiators, and added auxiliaries such as plasticizers, fillers, stabilizers etc.
The polymeric binders are usually thermoplastic elastomeric block copolymers, as disclosed in e.g. U.S. Pat. No. 6,531,263 and DE-C-2,215,090. These are generally block copolymers of the general formulae A-B-A or (AB)n or (AB)nX, comprising thermoplastic blocks A and elastomeric blocks B, particularly linear and radial block copolymers with poly(monovinyl aromatic hydrocarbon) end blocks.
Unhydrogenated styrene diene block copolymers, both the SIS and SBS types, can be used to make UV cured, flexographic printing plates. Although they are widely used and give excellent performance, one limitation is that the plates made with the unsaturated polymers have limited resistance to degradation by ozone. This is becoming more of a problem as UV cured inks become more widely used since ozone is generated from oxygen during the UV irradiation used to cure the ink. It would be highly desirable to make the plates using a blend of the unhydrogenated block polymer with at least a small amount of a hydrogenated block copolymer. The hydrogenated block polymer would probably not co-cure into the plate but it may improve the resistance to degradation by ozone. The problem is that hydrogenated block copolymers are typically thermodynamically incompatible with unhydrogenated block copolymers, and so blends are hazy or may even phase separate. The presence of haze in the plate would interfere with cure of the plate by scattering the UV light.
Now a novel anionic block copolymer based on mono alkenyl arene end blocks and controlled distribution mid blocks of mono alkenyl arenes and conjugated dienes has been discovered and is described in copending, commonly assigned U.S. patent application Ser. No. 60/355,210, entitled “NOVEL BLOCK COPOLYMERS AND METHOD FOR MAKING SAME”. Methods for making such polymers are described in detail in the above-mentioned patent application. Patentees have found that phase stable, clear compositions can be made using blends of unsaturated styrene/diene block copolymers, including both SIS and SBS copolymers, with these new polymers. This allows formulators to develop flexographic plates which have the good performance found for unhydrogenated block copolymers along with better resistance to degradation by ozone.